Area Forecast Discussion
Issued by NWS Goodland, KS

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000
FXUS63 KGLD 160443
AFDGLD

Area Forecast Discussion
National Weather Service Goodland KS
1043 PM MDT Tue Aug 15 2017

.UPDATE...
Issued at 1036 PM MDT Tue Aug 15 2017

Based on the elevated instability from the RAP, which has been the
most reasonable given the storm intensity, the threat for large
hail has ended. Elevated instability is less than 1000 j/kg where
the upper level lift is strongest. There will be some pockets of
higher elevated CAPE, but these will be far enough away from the
main upper level short wave trough that severe storms should not
develop. Some storms may have a hail core briefly reach severe
thresholds, but do not expect it to last long enough to produce
large hail. Storms have not lasted long when the hail core was
deep enough for large hail.

UPDATE Issued at 544 PM MDT Tue Aug 15 2017

Updated forecast to refine the timing of storms for tonight based
on latest data and satellite/radar trends. Main mechanism for
storm development will be the upper level short wave trough just
west of the Tri-State Area. This trough will deepen over a dry
line/surface trough just to the west. The upper level feature
will track northeast across the northwest half of the forecast
area tonight. With the strongest lift over this part of the
forecast area, the flash flood watch looks well placed. 600-500mb
frontogenesis will be strongest of Yuma County this evening, so
would expect a good deal of storm activity as a result. The
frontogenesis does weaken through the night and doesn`t move much
further east.

Meanwhile isentropic lift develops over the east third of the
forecast area overnight ahead of the upper level short wave
trough. Models have a second maximum of rainfall over this part of
the forecast area which is outside the watch. Confidence is not
high enough right now to justify extending the watch over that
part of the forecast area. However WPC does have a slight risk for
excessive rainfall over that part of the forecast area. Will
monitor new data coming in to see if the watch needs to be
expanded.

Regarding the risk for severe weather, deep layer shear is very
similar to yesterday as is the CAPE. The lift is even stronger
than yesterday, but the nose of the LLJ is not in as favorable a
position as yesterday. The nose is mainly over the northwest half
of the forecast area this evening, so would anticipate the severe
weather to be confined there for the most part. Flash flooding
should be the main threat with a secondary threat of hail and
damaging wind gusts. Cannot rule out tornadic development due to
0-1km helicity and 0-1km shear being favorable for tornadoes. The
tornadic threat will be confined to the first few hours of the
evening.

Once the cold front begins moving through late this evening,
anticipate the storm activity to decline behind the front.
Soundings show the elevated CAPE diminishing as the front moves
through.

On a side note, near term models not very optimistic with
rainfall over the majority of the forecast area tonight, which is
odd considering the depth of the lift with the upper level short
wave trough moving through. These models keep the storm activity
mostly north of the forecast area or over the far southern
counties. Cannot rule out greater storm activity over the south
than currently forecast, based on the latest radar trends, but
confidence is not as high as over the north half of the forecast
area where the upper level short wave trough will track tonight.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 247 PM MDT Tue Aug 15 2017

Forecast issues will coverage of thunderstorms and associated
hazards. Satellite and upper air analysis show a deep moist air
mass over the region. Main shortwave trough that will help with
the lift is just to our west at this time. Frontal boundary has
retreated some to the north through the day and is now over or
near the western and northern portions of the area.

High resolution/Cams are in disagreement where to put the convective
development which has been a problem they have been having this
season. At this time radar is showing thunderstorms developing near
the surface boundary. The Rap and latest Nam12 is showing this
scenario more than the other output. So at this time will start with
the highest pops in the north and west.

Guidance differs on how long to keep this activity around. Most of
the output is clustering around thunderstorms moving slowly east and
lasting through the night. Dcape looks to be 1000 or above over the
area through this evening and do have decent directional shear at
this time. Front and/or outflow moves across the area during this
time. So do expect a chance of severe with damaging wind and large
hail the main threats. There is an outside chance of a tornado.

Very heavy rainfall causing flash flooding looks to be higher than
normal. Precipitable water values are 1 to 1.5 inches which is near
to just above 1 to 1.5 standard deviations above normal. Locations
from Kit Carson through southeast Yuma and most of Sherman counties
into Cheyenne county Kansas and eastern Rawlins counties received to
2 to 6 inches with Kit Carson having the most widespread heavier
amounts. This combined where the models have the highest qpf and
pws, chose to put a Flash Flood Watch out for the northwest third.

For Wednesday the shortwave and associated precipitation moves
across the area through day and should be east of the area by late
in the afternoon. Models show cooler for tomorrow and used a blend
to cool temperatures off.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 205 PM MDT Tue Aug 15 2017

Over the extended period there will be a change in the upper level
pattern that will keep temperatures warmer and drier than the last
few weeks over the Tri-State region. To start off on Wednesday
night, precipitation chances are low as a shortwave trough moves east
of the region. As for Thursday, this is the best day during the
period that could see precipitation and storms. There is a broad
trough over the region and then a small shortwave passes over. This
will create uplift for storms to develop.

The remainder of the period will see ridging with southwest flow.
Mostly every day are showing signatures for precipitation with
moisture from the desert southwest being pulling into the region.

Temperatures are expected to remain the 80s for the period.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 544 PM MDT Tue Aug 15 2017

VFR to MVFR conditions forecast for the TAFs. Main focus is storm
arrival, with timing of both sites looking very similar.
Considered placing a tempo group for KGLD, but due to the site
being just south of the area of main storm activity did not. Did
include a mention of KMCK, which will be in a much better path for
storm activity. The storm activity will continue for KMCK into
the overnight hours. Heavy rainfall may drop the visibility to
IFR, but have low confidence of this occurring so only have MVFR
visibility. Storm activity should diminish behind the cold front
that will move through overnight, which will cause winds to turn
to the northwest. MVFR ceilings will move in for both sites
potentially, but definitely for KMCK toward 12z, lifting late
morning.


&&

.GLD WATCHES/WARNINGS/ADVISORIES...
KS...Flash Flood Watch until 6 AM MDT /7 AM CDT/ Wednesday for KSZ001-
     002-013.

CO...Flash Flood Watch until 6 AM MDT Wednesday for COZ090-091.

NE...Flash Flood Watch until 6 AM MDT /7 AM CDT/ Wednesday for
     NEZ079>081.

&&

$$

UPDATE...JTL
SHORT TERM...BULLER
LONG TERM...CLT
AVIATION...JTL


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